The driver should be considered for a tip too. On all the OAT trips that I have been on they recommended the following (below):
1) Program director (i.e. primarty tour guide) - $7 to $10 per person per day.
2) Driver - $1 to $2 per person for each 1/2 day and $3 to $4 per person for a full day.
Note: This has always been their standard recommendation regardless of the country. In my opinion it should be adjusted per country based on their standard of living.
Another note: Their recommendation for GCT trips are less due to the larger number of people per group, etc.